This invention relates to an apparatus for measuring the spatial variation of the magnetic field with signal processing techniques which permit the use of high sensitivity vector magnetometers such as super conducting quantum interference devices (SQUID). The apparatus uses a triad of gradiometers and filters to produce a high-pass filtered estimate of the field variation along the path.
The sensitivity of the instrument to temporal noise components is that of a short baseline gradiometer. The sensitivity function to spatial variation of ambient fields along the path of the platform can be structured into members of a family of desirable response functions. From this, a spatial field variation estimate can be derived which is relatively insensitive to small rotations of the magnetometer coordinate frame with respect to fixed geographic coordinates.
SQUID magnetometers offer much higher sensitivity than conventional magnetometers such as cesium vapor or proton precession instruments. The SQUID, however, is inherently a vector field sensor as compared to devices which sense only the absolute magnitude of the field. This vector property makes the SQUID difficult to use in a strong background field such that of the earth since a slight rotation of the instrument will change the projection of the strong background field on the sensitive axes of the instrument by an amount large compared to the small field variations one may wish to measure. Since the SQUID is by nature an incremental instrument, the output from each sensor contains a large and unknown offset. This precludes simple means of deriving rotation-insensitive spatial field estimates such as, for example, taking the square root of the sum of the squares of the outputs of three orthogonal sensors.
An approach which has been used in the past to reduce the sensitivity of vector magnetometers to noise fields in applications involving the location of ferrous objects utilizes measurements of field component gradients rather than the actual field components. An individual source of magnetic fields will be at least a dipole, and hence its field will decrease with the cube of distance. The gradient will decrease as the fourth power of distance.
For magnetic prospecting and applications involving the location of ferrous objects, the effective distance to disturbing noise sources will usually be large compared to the distance to the sources to be measured. Hence, a more favorable signal-to-noise ratio is obtained in a gradient measurement than in a direct field measurement. This will be particularly beneficial with respect to fluctuating terms due to sensor rotation. The disadvantage of this approach is that the signal field gradient is small with respect to the signal field and, even the ability to use sensitive SQUID magnetometers in forming the gradiometer may not recover this difference. This is the problem which is solved by the present invention.